VARIATION IN DIATOM COMMUNITY STRUCTURE AMONG HABITATS IN SANDY STREAMS1

Community structure of benthic diatoms in two sandy streams was studied for two summers to assess differences in abundances and species composition among habitats and between years and streams. The greatest differences in abundances (cells·cm^?2) were lower abundances on rocks than on sand, whether the sand was clean or covered with flocculent organic material. Relatively little variation in abundances occurred between years and streams. Species composition of diatom communities varied more among habitats and between streams than from year to year. Species composition was most unique in floc-covered sand when communities in clean and floc-covered sand, rock, plant, and plankton were compared. Diatom species composition in these sandy streams was most similar on sand and rocks.     

EFFECTS OF CURRENT VELOCITY AND LIGHT ENERGY ON THE STRUCTURE FO PERIPHYTON ASSEMBLAGE IN LABORATORY STREAMS1

Effects of current velocity and light energy on the taxonomica and physiognomic characteristics of periphyton assemblages were investigated in laboratory streams. The initial rate of colonization was related to current velocity, while the effects of light energy accounted for differences in species composition by the end of the experiment. Although the laboratory systems had many species in common during the realy stages of colonization, the experimental treatments generated differences in rates of communitydevelopment. synedra spp. were the early coloniters of the substrate, followed by an understory of Achnanthes spp. After day 16, Stigeoclonium tenue developed in the streams exposed to the higher photon flux density, but was rare in the shaded streams. The applicability of traditional successional theory to develoopmental patterns in lotic periphyton assemblages is discussed.     

PERIPHYTIC DIATOM ASSEMBLAGES FROM ULTRA‐OLIGOTROPHIC AND UV TRANSPARENT LAKES AND PONDS ON VICTORIA ISLAND AND COMPARISONS WITH OTHER DIATOM SURVEYS IN THE CANADIAN ARCTIC1

Periphytic diatoms are potentially powerful indicators of environmental change in climatically‐sensitive high latitude regions. However, only a few studies have examined their taxonomic and ecological characteristics. We identified and enumerated diatom assemblages from sediment, rock, and moss habitats in 34 ultra‐oligotrophic and highly transparent lakes and ponds on Victoria Island, Arctic Canada. The similar limnological characteristics of the sites allowed us to examine the influence of habitat, independent of water chemistry, on the diatom assemblages. As is typical in shallow arctic water bodies, benthic taxa, including species of Achnanthes, Caloneis, Cymbella, Navicula, and Nitzschia, were most widely represented. Minor gradients in our measured environmental variables did not significantly explain any variance in diatom species, but there were marked differences in diatom assemblages among sites. Pond ephemerality seems to explain some diatom variation, because aerophilic taxa such as Achnanthes kryophila Petersen and A. marginulata Grunow were dominant in shallow sites that had undergone appreciable reductions in volume. We identified several taxa that exhibited strong habitat preferences to sediment, moss, or rock substrates and also found significant differences (P < 0.01) in diatom composition among the three habitats. In comparisons with three similar diatom surveys extending over 1200 km of latitude, we determined that surface sediment assemblages differed significantly (P < 0.001) among all regions examined. Diatom species diversity was inversely related to latitude, a result likely explained by differences in the lengths of growing seasons. These data contribute important ecological information on diatom assemblages in arctic regions and will aid in the interpretation of environmental changes in biomonitoring and paleolimnological studies.     

INTERACTIONS BETWEEN OCEAN ACIDIFICATION AND WARMING ON THE MORTALITY AND DISSOLUTION OF CORALLINE ALGAE1

Coralline algae are among the most sensitive calcifying organisms to ocean acidification as a result of increased atmospheric carbon dioxide (pCO₂). Little is known, however, about the combined impacts of increased pCO₂, ocean acidification, and sea surface temperature on tissue mortality and skeletal dissolution of coralline algae. To address this issue, we conducted factorial manipulative experiments of elevated CO₂ and temperature and examined the consequences on tissue survival and skeletal dissolution of the crustose coralline alga (CCA) Porolithon (=Hydrolithon) onkodes (Heydr.) Foslie (Corallinaceae, Rhodophyta) on the southern Great Barrier Reef (GBR), Australia. We observed that warming amplified the negative effects of high pCO₂ on the health of the algae: rates of advanced partial mortality of CCA increased from <1% to 9% under high CO₂ (from 400 to 1,100 ppm) and exacerbated to 15% under warming conditions (from 26°C to 29°C). Furthermore, the effect of pCO₂ on skeletal dissolution strongly depended on temperature. Dissolution of P. onkodes only occurred in the high‐pCO₂ treatment and was greater in the warm treatment. Enhanced skeletal dissolution was also associated with a significant increase in the abundance of endolithic algae. Our results demonstrate that P. onkodes is particularly sensitive to ocean acidification under warm conditions, suggesting that previous experiments focused on ocean acidification alone have underestimated the impact of future conditions on coralline algae. Given the central role that coralline algae play within coral reefs, these conclusions have serious ramifications for the integrity of coral‐reef ecosystems.     

DISTURBANCE OF STREAM PERIPHYTON BY PERTURBATIONS IN SHEAR STRESS: TIME TO STRUCTURAL FAILURE AND DIFFERENCES IN COMMUNITY RESISTANCE1

The resistance of stream periphyton to structural disturbance by increases in shear stress (simulating a spate) was investigated in a laboratory flow tank. We monitored loss of biomass from a filamentous community (dominated by Melosira varians) under four different levels of shear stress. In each case, any loss that was going to occur did so within 10 min for this community. In a second experiment, we tested the resistance of four different communities (two dominated by nonfilamentous diatoms and two dominated by filamentous green algae/diatoms) to increases in shear stress. Nine different levels of shear stress were used, ranging from 1- to 70-fold higher than the conditions to which the communities were acclimated. All communities were 14 days old, but some differences in initial biomass occurred that influenced the degree of resistance independently of species composition. Overall, the nonfilamentous diatom communities were the most resistant, and the filamentous communities were the least resistant. The kinetics of the sloughing process varied among community types, with a community dominated by Melosira varians/Gom-phonema parvulum losing 50% of its biomass with only a 3-fold increase in shear stress. In contrast, a community dominated by the nonfilamentous diatoms Fragilaria vaucheriae/Cymbella minuta lost <50% of its biomass after a 70-fold increase in shear stress. Shear stresses required for 50% loss of biomass for the different communities were as follows: 3.6 Newtons.m^?2 for the Melosira varians/Gomphonema parvulum community, 10.0 N.m^?2 for the Spirogyra sp./Gomphoneis her-culeana/Ulothrix zonata community, 50.6 N.m^?2 for the Fragilaria construens/Cymbella minuta/Ach-nanthes minutissima community, and >90.0 N.m^?2for the Fragilaria vaucheriae/Cymbella minuta community. These results show that spates without bedload movement can potentially have widely differing disturbance effects on periphyton loss among streams depending on the initial taxonomic composition of resident communities. These results have important implications for stream ecosystem analysis and modeling.     

TESTING THE EFFECTS OF OCEAN ACIDIFICATION ON ALGAL METABOLISM: CONSIDERATIONS FOR EXPERIMENTAL DESIGNS1

Ocean acidification describes changes in the carbonate chemistry of the ocean due to the increased absorption of anthropogenically released CO₂. Experiments to elucidate the biological effects of ocean acidification on algae are not straightforward because when pH is altered, the carbon speciation in seawater is altered, which has implications for photosynthesis and, for calcifying algae, calcification. Furthermore, photosynthesis, respiration, and calcification will themselves alter the pH of the seawater medium. In this review, algal physiologists and seawater carbonate chemists combine their knowledge to provide the fundamental information on carbon physiology and seawater carbonate chemistry required to comprehend the complexities of how ocean acidification might affect algae metabolism. A wide range in responses of algae to ocean acidification has been observed, which may be explained by differences in algal physiology, timescales of the responses measured, study duration, and the method employed to alter pH. Two methods have been widely used in a range of experimental systems: CO₂ bubbling and HCl/NaOH additions. These methods affect the speciation of carbonate ions in the culture medium differently; we discuss how this could influence the biological responses of algae and suggest a third method based on HCl/NaHCO₃ additions. We then discuss eight key points that should be considered prior to setting up experiments, including which method of manipulating pH to choose, monitoring during experiments, techniques for adding acidified seawater, biological side effects, and other environmental factors. Finally, we consider incubation timescales and prior conditioning of algae in terms of regulation, acclimation, and adaptation to ocean acidification.     

THE DEVELOPMENT AND POTENTIAL ROLES OF CELL‐WALL TRABECULAE IN CAULERPA MEXICANA (CHLOROPHYTA)1

Siphonous plants represent an alternate scheme to the way most macroscopic plants are constructed. They are single, often large (1–2 m), sometimes morphologically complex, multinucleate (coenocytic) cells where the whole of the cytoplasm is a continuum. Caulerpa mexicana Sond. ex Kütz. is a siphonous tropical marine green alga characterized by four morphologically distinct regions and, as with other members of the genus, by the presence of a dense network of anastomosing cylindrical cell wall in growths called trabeculae. Based on the results of this study, we propose several roles for trabeculae: (i) They are structural components, which likely add some small amount of support in compression but add considerable strength in tension. (ii) As extensions of the cell wall and plasma membrane, they act as diffusion channels from the cell exterior to the interior cytoplasm. It is possible that trabeculae also play a role in determining cell shape through developmental positioning and placement patterns, thus facilitating the diverse shapes found in the morphologically distinct regions of Caulerpa sp.     

THE MECHANISMS OF FLUORIDE TOXICITY AND FLUORIDE RESISTANCE IN SYNECHOCOCCUS LEOPOLIENSIS (CYANOPHYCEAE)1

Fluoride was supplied as dissolved NaF at concentrations ranging from 0.26 to 7.9 mM (5–150 ppm) to three freshwater microalgae: Synechococcus leopoliensis (Racib.) Komarek (Cyanophyta), Oscillatoria limnetica Lemmermann (Cyanophyta) and Chlorella pyrenoidosa Chick (Chlorophyta). Growth of C. pyrenoidosa was unaffected by fluoride, and uptake of fluoride by this organism was not detectable. Growth of the cyanophytes was temporarily inhibited by NaF. The duration of this growth lag increased markedly as the pH was lowered at constant external fluoride concentration. In S. leopoliensis, fluoride uptake and inhibition of photosynthesis by NaF increased in the same way as did the growth lag in response to pH. Growth-inhibitory NaF treatments decreased the ATP level in cells of S. leopoliensis by 75% and also abolished phosphate uptake. Cells of S. leopoliensis in which fluoride-resistance was induced by prior growth in non-growth-inhibitory levels of NaF accumulated much less fluoride than did normal (“sensitive”) cells, and also did not respond to fluride by reduction of the ATP pool. It is suggested (1) that fluoride enters sensitive cells of S. leopoliensis principally as undissociated HF; (2) that its major inhibitory effect in these cells is the reduction in cellular ATP; (3)that fluoride-resistant cells accumulate less fluoride by developing incresed permeability to the fluoride anion.     

THE OPTIMUM PH OF FOUR STRAINS OF ACIDOPHILIC SNOW ALGAE IN THE GENUS CHLOROMONAS (CHLOROPHYTA) AND POSSIBLE EFFECTS OF ACID PRECIPITATION1

Four axenic strains of snow algae were examined for optimum pH under laboratory conditions using M-1 growth medium. Growth was measured using cell counts, cell measurements and absorbance readings at 440 nm. Strains C204 and C479A of Chloromonas sp. from the Adirondack Mountains, New York, grew optimally at pH 4.0 to 5.0. Strains C381F and C381G, Chloromonas polyptera (Fritsch) Hoh., Mull. & Roem. from the White Mountains, Arizona, grew optimally at pH 4.5 to 5.0. Growth was significantly higher at pH 4.0 in the northeastern species (Chloromonas sp.), but no significant difference was observed in final growth at pH 4.5, 5.0 and 5.5 between species. It is postulated that the more acidic precipitation in the northeastern United States may be selecting for strains of snow algae with greater tolerance to acidity than in strains from the southwestern United States or that the different pH optima reported are simply species differences. New York strain C204 was also grown in heavily buffered AM medium where it had an optimum pH of 5.0, but cells became irregularly shaped and tended to clump at pH 6.0 to 7.0. Growth of C204 in AM medium was significantly lower than in M-1 medium for snow algae. These findings justify the use of M-1 medium for this type of experimentation.     

THE ACCLIMATED RESPONSE OF GROWTH,PHOTOSYNTHESIS, COMPOSITION,AND CARBON BALANCE TO TEMPERATURE IN THE PSYCHROPHILIC ICE DIATOM NITZSCHIA SERIATA1

Nitzschia seriata Cleve, a common member of marine bottom ice communities in the Arctic, was grown in unialgal batch cultures to test for compensatory mechanisms for the low temperatures (?1.8° C) typical of its natural habitat. The upper lethal limit for growth was between 12° and 15°C, and the optimum was between 6° and 12° C. The Arrhenius function adequately (R²= 73%) fitted the relationship between growth rate and temperature from – 1.6° up to 10° C, with an average Q₁₀ of 1.9 over the entire range. Light-saturated and light-limited rates of photosynthesis (normalized to chlorophyll a or cell carbon) showed complete compensation from 12° to 4° C. Photosynthetic rates, especially at light saturation, declined rapidly at temperatures below 4° C. Susceptibility to photoinhibition was greatest at the lowest growth temperatures. Cellular composition (chlorophyll a, protein, polysaccharide, and lipid contents) was not systematically related to temperature in any simple way, although cell size (carbon per cell) was maximal at the lowest growth temperature. Dark respiration was unmeasurably low (<0.015 day^?1) at all growth temperatures. The strategy of adaptation in N. seriata may be characterized as optimizing efficiency and compensation, rather than maximization, of growth rate.     

STREAM PERIPHYTON PHOTOACCLIMATION RESPONSE IN FIELD CONDITIONS: EFFECT OF COMMUNITY DEVELOPMENT AND SEASONAL CHANGES1

The photochemical behavior of intact stream periphyton communities in France was evaluated in response to the time course of natural light. Intact biofilms grown on glass substrata were collected at three development stages in July and November, and structural parameters of the biofilms were investigated (diatom density and taxonomy). At each season, physiological parameters based on pigment analysis (HPLC) and pulse‐amplitude‐modulated (PAM) chl fluorescence technique were estimated periodically during a day from dawn to zenith. Regardless of the community studied, the optimal quantum yield of PSII (F_v/F_m), the effective PSII efficiency (Φ_PSII), the nonphotochemical quenching (NPQ), and the relative electron transport rate (rETR) exhibited clear dynamic patterns over the morning. Moreover, microalgae responded to the light increase by developing the photoprotective xanthophyll cycle. The analysis of P‐I parameters and pigment profiles suggests that July communities were adapted to higher light environments in comparison with November ones, which could be partly explained by a shift in the taxonomic composition. Finally, differences between development stages were significant only in July. In particular, photoinhibition was less pronounced in mature assemblages, indicating that self‐shading (in relation to algal biomass) could have influenced photosynthesis in older communities.     

DISTRIBUTION OF THE GENUS CYCLOTELLA IN RELATION TO pH IN PRECAMBRIAN SHIELD LAKES: IMPLICATIONS FOR DIATOM-INFERRED pH1

Recent research on relationships between diatoms and pH suggests that the genus Cyclotella exhibits a strong relationship with lake acidity, being almost totally absent below pH 5.5. This decline has been used as an indicator of lake acidification in paleolimnological studies. In this study C. stelligera V.H. and C. kützingiana Thwaites were abundant in Precambrian Shield lakes with pH as low as 4.5. Cyclotella comta (Ehr.) Kütz. was found in lakes of pH < 5.5, but maximum abundance was observed in lakes of pH > 5.5. Cyclotella michiganiana Skv. was found in lakes of pH < 6.0. These results indicate that the use of C. stelligera, C. kützingiana, and possibly C. comta, in paleolimnological investigations of lake acidification, should be approached with caution. These taxa may exhibit a decline in abundance with decreasing lakewater pH, but this is partially a morphometric effect not necessarily related to anthropogenic acidification.     

CARBON‐USE STRATEGIES IN MACROALGAE: DIFFERENTIAL RESPONSES TO LOWERED PH AND IMPLICATIONS FOR OCEAN ACIDIFICATION1

Ocean acidification (OA) is a reduction in oceanic pH due to increased absorption of anthropogenically produced CO₂. This change alters the seawater concentrations of inorganic carbon species that are utilized by macroalgae for photosynthesis and calcification: CO₂ and HCO₃^? increase; CO₃^2? decreases. Two common methods of experimentally reducing seawater pH differentially alter other aspects of carbonate chemistry: the addition of CO₂ gas mimics changes predicted due to OA, while the addition of HCl results in a comparatively lower [HCO₃^?]. We measured the short‐term photosynthetic responses of five macroalgal species with various carbon‐use strategies in one of three seawater pH treatments: pH 7.5 lowered by bubbling CO₂ gas, pH 7.5 lowered by HCl, and ambient pH 7.9. There was no difference in photosynthetic rates between the CO₂, HCl, or pH 7.9 treatments for any of the species examined. However, the ability of macroalgae to raise the pH of the surrounding seawater through carbon uptake was greatest in the pH 7.5 treatments. Modeling of pH change due to carbon assimilation indicated that macroalgal species that could utilize HCO₃^? increased their use of CO₂ in the pH 7.5 treatments compared to pH 7.9 treatments. Species only capable of using CO₂ did so exclusively in all treatments. Although CO₂ is not likely to be limiting for photosynthesis for the macroalgal species examined, the diffusive uptake of CO₂ is less energetically expensive than active HCO₃^? uptake, and so HCO₃^?‐using macroalgae may benefit in future seawater with elevated CO₂.     

海水酸化及升温对刺参生长及能量收支的影响

以东亚浅海生态系统中的关键种——刺参(Apostichopus japonicus)为实验对象,研究了CO₂驱动的海水酸化及升温对其生长及能量收支的影响。实验设置对照组(大连近海水温, pCO₂ 400μatm)、升温组(大连近海水温+3℃, pCO₂ 400μatm)、酸化组(大连近海水温, pCO₂ 1100μatm)和酸化升温组(大连近海水温+3℃,pCO₂ 1100μatm)。结果表明:与对照组相比,温度升高3℃对刺参的生长无显著影响;酸化组刺参的特定生长率最低,较对照组降低0.19%/d,个体体重的变异系数最大;酸化升温组刺参的终末体重和特定生长率与对照组相较无显著差异,但其摄食率和排粪率均显著高于对照组。升温组和酸化组的刺参能量的分配模式与对照组相比未发生明显改变,但酸化升温组刺参的能量分配模式发生显著变化,其粪便能所占摄食能的比例显著升高。研究表明,海水酸化抑制了刺参的生长但未改变其能量的分配,生长的降低主要取决于摄食减少;而海水酸化与温度升高的共同作用可能会通过...     

ALGAL COMMUNITIES OF SPRINGS AND STREAMS IN THE MT. ST. HELENS REGION,WASHINGTON, U.S.A. FOLLOWING THE MAY 1980 ERUPTION1

Mt. St. Helens, a volcanic peak in the Cascade Range in southern Washington erupted violently on May 18, 1980, causing enormous damage to both terrestrial and aquatic ecosystems. The initial explosion evaporated, scoured or buried all springs and streams in the blast impact area. Ash fall and erosion from defoliated hillsides subsequently filled most of the lotic habitats with organic debris and volcanic ash. Recolonization of springs and streams by algae occurred quickly in areas where erosion through the ash progressed down to bedrock. Within 15 months or less of the eruption, algal communities were established throughout the blast impact area. However, as a result of the initial and continued disturbance these communities remained in an early successional stage. Floral assemblages were highly variable except that they were composed mostly of diatoms, with Achnanthes minutissima dominating most lotic sites. Springs showed the most rapid development toward stable floras.     

PRESENCE AND DIVERSITY OF ALGAL TOXINS IN SUBTROPICAL PEATLAND PERIPHYTON: THE FLORIDA EVERGLADES,USA1

Production of toxic secondary metabolites by cyanobacteria, collectively referred to as cyanotoxins, has been well described for eutrophied water bodies around the world. However, cohesive cyanobacterial mats also comprise a significant amount of biomass in subtropical oligotrophic wetlands. As these habitats generally do not support much secondary production, cyanotoxins, coupled with other physiological attributes of cyanobacteria, may be contributing to the minimized consumer biomass. Periphyton from the Florida Everglades has a diverse and abundant cyanobacterial assemblage whose species produce toxic metabolites; therefore, by screening periphyton representative of the greater Everglades ecosystem, six different cyanotoxins and one toxin (domoic acid) produced by diatoms were identified, ranging in content from 3 × 10^?9 to 1.3 × 10^?6 (g · g^?1), with saxitoxin, microcystin, and anatoxin‐a being the most common. While content of toxins were generally low, when coupled with the tremendous periphyton biomass (3–3,000 g · m^?2), a significant amount of cyanotoxins may be present. While the direct effects of the toxins identified here on the local grazing community need to be determined, the screening process utilized proved effective in showing the broad potential of periphyton to produce a variety of toxins.     

EVIDENCE FOR KINESIN‐ AND DYNEIN‐LIKE PROTEIN FUNCTION IN CIRCULAR NUCLEAR MIGRATION IN THE GREEN ALGA PLEURENTERIUM TUMIDUM: DIGITAL TIME LAPSE ANALYSIS OF INHIBITOR EFFECTS1

The unicellular green alga Pleurenterium tumidum Bréb. performs a unique type of circular nuclear migration, wherein the nucleus leaves its central position and starts revolutions in the cortical isthmus area about 10 h after mitosis. This motion lasts for at least 12 h with an average velocity of about 1 h per revolution. Possible force generation modes during circular nuclear migration of Pleurenterium were investigated by application of inhibitors and the use of digital time‐lapse video microscopy. 5′‐Adenylylimidodiphosphate, a nonhydrolyzable nucleotide analogue, retarded or inhibited circular nuclear migration, suggesting that ATPase dependent motor proteins are involved. Ado‐ ciasulfate‐2, a kinesin specific inhibitor, caused displacement of the nucleus, suggesting that the linkage between the microtubule track and the nucleus is lost. The nucleus was still able to move for short distances, but no normal revolutions took place. Erythro‐9‐[3‐(2‐hydroxynonyl)] adenine, a dynein ATPase inhibitor, led to complete inhibition of nuclear revolutions, suggesting a function in force generation also for this molecular motor. In addition, kinesin‐ and dynein‐like proteins were detected in Pleurenterium extracts by Western blotting. The myosin specific inhibitor 2,3‐butanedione 2‐monoxime did not influence circular nuclear migration in Pleurenterium. This result and the absence of actin filaments around the migrating nucleus as depicted by means of microinjection of Alexa phalloidin in the present study indicate that the actin‐myosin system can be excluded from force generation.     

ULTRASTRUCTURE OF CELL DIVISION AND REPRODUCTIVE DIFFERENTIATION OF MALE PLANTS IN THE FLORIDEOPHYCEAE (RHODOPHYTA): CELL DIVISION IN POLYSIPHONIA1

Cell division in the marine red algae Polysiphonia harveyi Bailey and P. denudata (Dillwyn) Kutzing was studied with the electron microscope. Cells comprising the compact spermatangial branches of male plants were used exclusively because of their small size, large numbers and the ease with which the division planes can be predetermined. Some features characterizing mitosis in Polysiphonia confirm earlier electron microscope observations in Membranoptera, the only other florideophycean algae in which mitosis has been studied in detail. Common to both genera are a closed, fenestrated spindle, perinuclear endoplasmic reticulum, a typical metaphase plate arrangement of chromosomes, conspicuous, layered kinetochores, chromosomal and non-chromosomal microtubules, and nucleus associated organelles (NAOs) known as polar rings (PRs) located singly in large ribosome-free zones of exclusion at division poles in late prophase. However, other features, unreported in Membranoptera, were observed consistently in Polysiphonia. These include the presence of PR pairs in interphase-early prophase cells, the attachment of PRs to the nuclear envelope during all mitotic stages, the migration of a single PR to establish the division axis, a prominent, nuclear envelope protrusion (NEP) at both division poles at late prophase, the prometaphase splitting of PRs into proximal and distal portions, and the reformation of post-mitotic nuclei by the separation of an elongated interzonal nuclear midpiece at telophase. During cytokinesis, cleavage furrows impinge upon a central vacuolar region located between the two nuclei and eventually pit connections are formed in a manner basically similar to that reported for other red algae. Diagrammatic sequences of proposed PR behavior during mitosis are presented which can account for events known to occur during cell division in Polysiphonia. Mitosis is compared with that reported in several other lower plants and it is suggested that features of cell division are useful criteria to aid in the assessment of phylogenetic relationships of red algae.